1. Field of the Invention.
The present invention provides a method for cleaning a surface of a silicon wafer, and more particularly, a method for cleaning the surface of a wafer with a line width below 0.15 micrometers.
2. Description of the Prior Art
In integrated circuit fabrication processes, a wafer has to repeatedly go through various chemical processes, such as chemical vapor deposition (CVD), dry etch, or chemical mechanical polishing (CMP), etc., which leave contaminating particles on a surface of the wafer. Moreover, a surface of the substrate is easily oxidized and forms a native oxide layer, which effects quality of the wafer product. For these reasons, a wafer has to be cleaned several times during processing in order to remove the contaminates and the native oxide layer, so as to maintain surface cleanliness.
In general, there are two types of cleaning methods, dry and wet. The wet cleaning method is used more frequently. The wet cleaning method employs an RCA cleaning technique, which makes use of the following solutions:
1) RCA Standard Clean 1 (SC-1), which can remove the micro-particles left on the surface of a wafer to be cleaned, and is made of one part NH4OH, one part H2O2, and five parts H2O, and in which the wafer is immersed for 5 minutes at 70xc2x0 C.
2) RCA Standard Clean II (SC-2), which can remove the metallic particles left on a wafer""s surface, and is made of one part HCl, one part H2O2, and six parts H2O, and in which the wafer is immersed for 5 to 10 minutes at 70xc2x0 C.
3) Piranha (SPM), which can remove the organic particles from the surface of the wafer, is made of five parts H2SO4 and one part H2O2, and in which the wafer is immersed for 10 to 15 minutes at 120xc2x0 C.; and
4) Diluted hydrofluoric acid clean (DHF), which is used to remove the silicon dioxide layer from the surface of the wafer, and is made of one part HF and fifty parts H2O, and in which the wafer is immersed for 10 to 30 seconds at room temperature.
Recently, in order to save cleaning agents and improve cleaning quality, other cleaning agents than the above mentioned have been developed. For example, an O3/H2O solution which, being a good oxidizing agent, can remove organic particles and metallic particles at room temperature, is developed to replace SPM, which works well only at the temperature above 120xc2x0 C. Moreover, surfactant and ultrasonic oscillation techniques are often used with the conventional cleaning processes, so as to achieve optimal cleaning results.
Please refer to FIG. 1. FIG. 1 is a flow chart of a cleaning process 10 for cleaning wafers at room temperature according to the prior art. As shown here, the cleaning process 10 comprises the following steps:
Step 12: Immersing the wafer to be cleaned in a solution of O3 in water (O3/H2O), as O3 is a good oxidizing agent, to remove hydrocarbons, other organic particles, and inactive metallic ions, such as Au and Ag.
Step 14: Immersing the wafer in an HF/H2O2 solution to remove metallic ions and a native oxide layer, and, after reducing adhesiveness of the micro-particles to the surface of the wafer with a surface activator, employing the ultrasonic oscillating technique to remove the loosened particles.
Step 16: Cleaning the surface of the wafer with ultra-pure water (UPW) to remove remaining inorganic particles left on the surface of the wafer after Step 14.
Step 18: Immersing the wafer in a solution of O3 in water to remove the surface activator left on the surface of the wafer resulting from step 14.
Step 20: Using DHF solution to remove from the surface of the wafer the native oxide layer resulting from Step 18.
Step 22: Using UPW to clean the surface of the wafer.
Because of demand on the density of devices on the wafer, and with the development of photo printing techniques, the widths of contact holes, vias or trenches on the surface of a wafer are getting smaller, and the aspect ratios are getting larger. For example, a contact hole 0.15 mm in width with an aspect ratio of 10 would have a depth of 1.5 mm. For this reason, the cleaning agents can not easily reach the bottom of the contact hole under capillary attraction, resulting in a reduction in cleaning efficiency, affecting the rest of the manufacturing process, and ultimately the quality of the final product.
Please refer to FIG. 2. FIG. 2 is a cross-sectional view of a wafer 100 undergoing the cleaning processes. As shown, the wafer 100 is immersed in a cleaning agent 200, and the surface of the wafer 100 comprises a contact hole 102 of width 0.15 mm, a contact hole 104 of width 0.18 mm, a contact hole 106 of width 0.3 mm, and a contact hole 108 of width 0.5 mm, wherein their aspect ratios are 6, 10, 3 and 1, respectively. Unlike the contact holes 106 and 108, the contact holes 102 and 104 have widths less than 0.2 mm, so air chambers 202, due to capillarity, are formed at the bottoms of the contact holes 102 and 104, and prevent the cleaning agent 200 from reaching the bottoms of the contact holes 102 and 104, reducing the cleaning efficiency for the wafer 100.
The main purpose of the present invention is to provide a cleaning method for cleaning a surface of a wafer comprising contact holes, a via, or trenches less than 0.2 mm in width.
In this method, a wafer is placed in a closed cleaning chamber, and then the cleaning agent is poured into the cleaning chamber to a predetermined height, so that the wafer is completely immersed in the cleaning agent. Then, the pressure in the cleaning chamber is lowered to a sub-atmospheric statexe2x80x94between 0.1 and 0.9 atm. To finish, the pressure is returned to a normal value. The cleaning agent can be dilute hydrofluoric acid (DHF), ultra-pure water (UPW), RCA standard clean 1 (SC-1), RCA standard clean 2 (SC-2), a solution of O3 in water (O3/H2O), a solution of HF in H2O2, buffered oxide etchant (BOE), or a hot phosphoric acid solution.
In the present invention, because the pressure in the cleaning chamber is lowered to a sub-atmospheric state in a few seconds, the cleaning agent can thoroughly clean the surface of a wafer that has contact holes or trenches with large aspect ratios, resulting in a good cleaning quality. Also, since the contact holes with large aspect ratios can be wetted in a very short period of time, the cleaning and etching times can be precisely controlled.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.